Protection of blast holes

ABSTRACT

A method of protecting predilled, uncharged bore holes for use in an underground mine engaged in stope mining; the method comprising the steps of filling the uncharged bore holes with a liquid and sealing the entry of the liquid-filled blast holes prior to detonating adjacent blast holes. The liquid to be used will depend on the material being mined. In impervious material, the liquid is water. In porous material, the liquid may be viscous or have sealing properties such as a drilling mud or bentonite mud. Various plugs for sealing the bore holes are also described and claimed.

FIELD OF THE INVENTION

[0001] The present invention relates generally to the art of blasting. It has particular application to preparation of blast holes

BACKGROUND ART

[0002] It is established practice in the preparation for blasting operations to drill in at the same time a series of sets of blast holes which are to be successively filled with explosive and detonated. The holes that are left empty for subsequent blasting operations often become damaged by the earlier blasting operations and must be repaired prior to being filled with explosive. This repair work causes inconvenience and additional expense to the blasting operation.

[0003] Throughout this specification, the term liquid shall be taken as comprising any liquid, slurry or material having liquid like properties.

DISCLOSURE OF THE INVENTION

[0004] As described below, the present invention discloses a method of protecting a predrilled, uncharged blast hole comprising the step of substantially filling the blast hole with a liquid prior to blasting adjacent holes.

[0005] Accordingly, the present invention resides in a method of protecting a predrilled, uncharged blast hole comprising the steps of substantially filling the blast hole with a liquid before adjacent blasting is conducted and sealing the entry of the blast hole.

[0006] According to a preferred feature of the invention, the entry of the blast hole is sealed by inserting a plug into the entry of the blast hole and positioning the plug adjacent to the entry of the blast hole.

[0007] According to a preferred feature of the invention, liquid is delivered into the blast hole after the plug is inserted.

[0008] According to a preferred feature of the invention, the entry of the blast hole is lined before it is sealed by the plug.

[0009] Accordingly, the invention also resides in a plug adapted to seal a blast hole against leakage of liquid comprising sealing means adapted to provide sealing engagement between the bore of the blast hole and the plug, and retaining means adapted to positionally retain the plug within the blast hole.

[0010] According to a preferred feature of the invention, the sealing engagement is releasable.

[0011] According to a preferred embodiment, the sealing means is adapted to provide sealing engagement between the bore of the blast hole and the plug by expansion of the sealing means.

[0012] According to a preferred embodiment, the sealing means is responsive to pressure to provide sealing engagement. According to a preferred embodiment, the sealing means comprises a seal seated in a groove. According to a preferred embodiment, the sealing means comprises a seal having a ‘U’ shaped cross-section. According to a preferred embodiment, the sealing means comprises a seal having a circular cross-section.

[0013] According to a preferred embodiment, the retaining means is adapted to retain the plug within the blast hole by releasable engagement of the retaining means with the bore of the blast hole.

[0014] According to a preferred embodiment, the engagement of the retaining means with the bore of the blast hole is effected by expansion of the retaining means.

[0015] According to a preferred embodiment, the retaining means, when in use, is adapted to provide additional retaining force to the bore of the blast hole when additional force is applied to the plug from within that portion of the blast hole enclosed by the plug.

[0016] According to a preferred embodiment, the retaining means comprises an annular member received around the plug, the annular member having a plurality of axially spanning engagement ribs surrounding the periphery of the annular member means. According to a preferred embodiment, the annular member comprises a plurality of part-annular segments.

[0017] According to a preferred embodiment, the inner surface of the retaining means is tapered.

[0018] According to a preferred feature of the invention, the plug is associated with a supply duct adapted to allow the passage of liquid past the plug.

[0019] According to a preferred feature of the invention, the supply duct is provided through the plug.

[0020] According to a preferred embodiment, the plug is further provided with pressure-communication means adapted to communicate pressure from the supply duct to the sealing means.

[0021] According to a preferred embodiment, the supply duct is provided with a pressurising nozzle adapted to restrict the flow of liquid through the nozzle and thereby ensure pressure is provided within the supply duct during filling of the blast hole with liquid.

[0022] According to a preferred feature of the invention, the plug is associated with a breather duct adapted to allow the passage of air past the plug.

[0023] According to a preferred feature of the invention, the breather duct is provided through the plug.

[0024] According to a preferred embodiment, the plug further comprises a bleed hole adapted to provide re-sealable communication between the breather duct and an external surface of the plug.

[0025] According to one embodiment, the tube engages the breather duct. According to an alternative embodiment, the tube engages the supply duct.

[0026] According to a preferred feature of the embodiment, the external surface of the main body is tapered.

[0027] According to a preferred feature of the embodiment, the tapered surface of the main body is adapted to cooperate with the tapered surface of the retaining means to provide expansion of the retaining means.

[0028] According to a preferred embodiment, the plug is re-usable.

[0029] According to a preferred embodiment, the plug is disposable.

[0030] The invention will be more fully understood in the light of the following description of several embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The description is made with reference to the accompanying drawing of which:

[0032]FIG. 1 is a diagrammatic elevation view of a mine site wherein an embodiment of the invention may be utilized;

[0033]FIG. 2 is a sectional view of the mine site illustrated in FIG. 1 though Section A-A diagrammatically illustrating a typical arrangement of longholes emanating from a point in the mine tunnel;

[0034]FIG. 3 is a sectional elevation view of a longhole in which a plug is positioned in accordance with a preferred embodiment;

[0035]FIG. 4 is a sectional view of a plug in accordance with a preferred embodiment;

[0036]FIG. 5 is a cross-section of the breather hole through section line B-B of the embodiment as shown in FIG. 4;

[0037]FIG. 6 is an isometric view of engagement means of a plug in accordance with the embodiment as shown in FIG. 4.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

[0038] An example of the method of protecting a blast hole in accordance with the invention is now described with reference to FIGS. 1, 2, 3, 4, 5 and 6. For the purpose of clarity, the embodiment is described with reference to its application to the art of underground stope mining techniques. In this application, a blast hole is referred to as a longhole and this term will be used henceforth in relation to the description of this type of blast hole.

[0039]FIG. 1 depicts a diagrammatical cross-sectional view of an underground mine site engaged in stope mining in a conventional manner. The mine site comprises an ore body 11 identified for removal, at which preliminary mining has been undertaken to establish a substantially vertical ore face 12, hereinafter referred to as a vertical ore face. The excavated region in front of the vertical ore face 12 is termed the gallery 13. Access is obtained to the gallery 13 by a lower tunnel 14 driven below the ore body 11. Ore is removed from the ore body 11 by blasting successive sections of ore from the vertical ore face 12, thereby enlarging the gallery 13 and creating a new vertical ore face with each blasting operation. Typically, the height of ore mined in this way in a single blasting operation is in the range of 10 to 50 metres. The depth of each section removed in a single blasting operation depends on the ore being mined. After blasting, the ore is removed from the gallery 13 by vehicles which gain access through the tunnel. Where the ore body extends to a substantial depth, the ore is mined in a series of vertical layers in the above-described manner.

[0040] As depicted in FIG. 2, the preparation for blasting comprises the step of drilling a group of longholes from a location along the tunnel in an arrangement that fans upwardly into the ore from the tunnel roof. The group of longholes are coplanar in a plane parallel to the vertical ore face 12. Each location in the tunnel at which a group of longholes is drilled is termed a ring. After drilling, the longholes are filled with explosive and detonated to blast free the ore between the ring and the vertical ore face 12.

[0041] Longholes are drilled by rock drills mounted on a vehicle known as a longhole drill rig. It is common practice to drill longholes at a series of rings along the length of a tunnel before blasting is commenced. This is partly due to costs of operating the longhole drill rig and general mining efficiencies. Usually, only the longholes of the ring nearest the vertical ore face 12 are filled with explosive and detonated in one blasting operation. In some circumstances, longholes of a plurality of rings will be blasted at once. The remaining longholes of rings further back from the vertical face are left uncharged during adjacent blasting. It is common that the uncharged longholes adjacent those that are exploded are damaged by the neighbouring explosion. Damaged longholes must be repaired by re-drilling or other means and the repair work adds cost to the mining operation.

[0042] The present embodiment discloses a method of protecting the predrilled, uncharged longholes which has been devised to eliminate or at least reduce the damage to the longholes as mentioned above. This method comprises the process of substantially filling the longholes with a liquid prior to detonating an explosion in longholes of a neighbouring ring or rings. The liquid to be used will depend upon the circumstances of the ore to be mined. Where the longhole is drilled in impervious material, the preferred liquid will generally be water due to its availability and ease of use. However, where the ore is porous or contains cracks, an alternative liquid, having viscous or sealing properties, may be required. In some ores, drilling mud or bentonite mud is found to be suitable due to its availability and its tendency to seal porous or cracked ore sufficiently for the purposes of the embodiment.

[0043] According to the embodiment, the longhole is sealed by a plug before the longhole is filled with liquid. An embodiment of a preferred plug is shown in FIGS. 4, 5 and 6. FIG. 4 illustrates a sectional view of the embodiment and shows a plug comprising a main body (201) and a separate retaining means (202).

[0044] The main body (201) comprises a body having a first end (202), a second end (203), a first cylindrical portion (212), a second cylindrical portion (221) and a tapered portion (222). The main body further comprises a liquid supply duct (214) and a breather duct (215), which ducts extend longitudinally through the main body from the first end (202) to the second end (203).

[0045] The first cylindrical portion (212) extends from the first end of the main body and comprises a cylindrical periphery (216) having a diameter adapted to be slightly smaller than the diameter of the longhole in which the plug is to be used to enable the main body to be inserted and guided within the longhole without difficulty. Intermediate the cylindrical periphery (216) is a groove (217) adapted to receive a seal (218). The seal (218) comprises a circular seal body having a U section, and is adapted to be expanded by pressure into sealing engagement with the bore of the longhole. The seal (218) is provided with an outer surface adapted to engage the bore of the longhole, thereby providing sealing engagement, and an inner surface opposed to the outer surface. The plug is further provided with a pressurizing hole which extends from the supply duct to the groove (217). Pressure is applied to the inner surface of the seal from the supply duct through the pressurizing hole (219).

[0046] The second cylindrical portion (221) adjoins the first cylindrical; portion (212) and has a reduced diameter from that of the first cylindrical portion (212). The tapered portion (222) extends from the second cylindrical portion (221) and is provided with a taper reducing in diameter towards the second end (202) of the main body. The edge of the tapered portion (213) at the second end (202) of the main body is chamfered.

[0047] The supply duct (214) is threaded at each end. The end of the supply duct (214) at the first end (202) of the main body is intended to be engaged by a pressurizing nozzle (220).

[0048] The breather duct (215) is adapted to receive and seal a breather tube (110) by having an opening (223) at the first end (202) of the main body of enlarged diameter. Sealing means (224) in the form of an O-ring is provided within the enlarged opening and to sealingly engage the breather tube (110).

[0049] A threaded end of the breather duct (215) is provided at the second end (203) of the main body. A bleed hole (225) enters the breather duct intermediate the length of the thread of the threaded end. A bleed duct (226) extends from the second end (203) of the main body to the bleed hole (225) to provide a passageway between the breather duct (215) and second end (203) of the main body. The threaded end of breather duct (215) is adapted to be threadably engaged by a rod (108) to provide sealing of the breather duct when the rod (108) is fully engaged in the threaded end and to provide communication from the breather tube, through the bleed hole (225) and bleed duct (226) to the second end of the main body when the rod (108) is disengaged sufficiently to open the bleed hole (225).

[0050] As shown in FIG. 6, the retaining means (111) comprises a pair of part-annular segments (112), each of which is provided with an outer surface (113), an inner surface (114), a first radial face (115), a second radial face (116), an upper end (117) and a lower end (118). The outer surface (113) comprises a plurality of axially spaced engagement ribs around the periphery of the part-annular segment (112).

[0051] Each engagement rib is provided with an engagement edge (124) adapted to retainingly engage the bore of the longhole.

[0052] The first radial face (115) of each part-annular segment is provided with at least one tab (131) circumferentially extending from the first radial face (115). The second radial face (115) is provided with at least one slot (132) disposed at a position along its length corresponding to the position of the tab (131) and adapted to be loosely engaged by the corresponding tab (131) of the second of the pair of part-annular segments (111).

[0053] In use, the two part-annular segments (112) are positioned together, the first radial face (115) of a one part-annular segment adjoining the second radial face (116) of the other part-annular segment, the tab (131) of the one part-annular segment loosely engaging the slot (132) of the other part-annular segment, thereby preventing relative longitudinal movement between the two part-annular segments (112) during insertion in the longhole. During insertion, the two part-annular segments are held together by resilient means such as brings placed around the outer surface (113).

[0054] As shown in FIG. 4, the inner surface (114) of the each part-annular segment (112) has two portions, an first portion (141) of regular profile which, when disposed with the corresponding second part-annular segment, provides a cylindrical profile, and a second portion (142) which, when disposed with the corresponding second part-annular segment, provides a tapered profile. The angle of taper of the second portions (142) of the part-annular segments corresponds with the angle of taper of the tapered portion (222) of the main body.

[0055] The use of the plug (102) is more clearly shown in FIG. 3. The plug (102) is inserted a short depth into the longhole and positioned adjacent to the entry of the long hole. Usually the depth of insertion is in the order of 1 metre.

[0056] The supply duct (214) and the breather duct (215) provide communication between the tunnel and the portion of the longhole enclosed by the plug (102). In use, a supply pipe (106) threadably engages the supply duct (214) at the second end (203) of the main body (201) and extends into the tunnel. A stop-valve (107) is provided on the supply pipe (106) adjacent the end within the tunnel. A supply hose (103) having a valve (104) is connected to the supply pipe (106) to enable liquid to be delivered into the longhole by opening valve (104) and stop-valve (107),

[0057] A breather tube (110) is sealingly received in the opening (223) of the breather duct (215) and extends into the longhole to adjacent the inner end (100) of the longhole (101) opposed to the plug (102). A rod (108) engages the threaded end of the breather duct (215).

[0058] During the process of insertion of the plug into the longhole and filling thereof with liquid, the breather tube (110) of appropriate length is first inserted into the longhole. The breather tube (110) has at least some flexibility enabling it to be coiled yet is sufficiently rigid so that it will not buckle while being inserted into the longhole. Polythene tubing of the type used in domestic reticulation systems is found to be particularly suitable. The end adjacent the entry of the longhole is inserted into the breather duct in the main body (102), being sealed by the O-ring (224). Supply pipe (106) and rod (108) are attached to the main body (102). The pair of part-annular segments (112) of the retaining means (111) are arranged in relation to the main body (102) and retained by O-rings (122) around the periphery of the part-annular segments. Thereafter, the assembly is inserted into the longhole to an appropriate distance. The valve (104) of supply hose (103) is opened and liquid passes through the supply pipe (106) into the supply duct (214). The liquid passes through the pressurizing nozzle (220) which has an opening smaller in diameter than the diameter of the supply duct (214), thereby ensuring a pressure is maintained within the supply duct (214) during the initial phase of filling. Pressure is applied to the inner surface of the seal (218) through pressurising hole (219), thereby urging the outer face of seal (218) into contact with the bore of the longhole and ensuring sealing engagement during the filling process. After filling is completed, pressure is maintained upon the seal by the pressure from the head of liquid within the longhole.

[0059] During the filling process, as liquid enters the longhole and progressively fills it, air escapes through the breather tube (110), breather duct (215), bleed hole (225) and bleed duct (226). When the longhole is filled to the level of the inner end of the breather tube, liquid will be expelled through the breather tube (110) instead of air, indicating to the operator to close valve (104) of supply hose (103), valve (107) of supply pipe (106) and screw rod (108) into sealing engagement with breather duct (215) to close bleed hole (225). The supply hose (103) may then be disconnected.

[0060] When it is time to fill the longhole with explosive, the liquid is removed from the longhole by opening the valve (107) of supply pipe (106) as well opening breather duct (215) by unscrewing rod (108) past the breather hole (225). The liquid escapes under its own weight while air enters the longhole through the bleed duct (226), bleed hole (225) and breather duct (215). The plug is removed from the longhole by inwardly tapping the main body (201) from the tunnel, thereby tending to move it out of engagement with the retaining means (111). Meanwhile, the O-rings (122) urge the two part-annular segments together to cease their engagement with the bore of the longhole, and thus enabling removal.

[0061] In certain ore conditions, it is found appropriate to first line the base of the longhole to the depth of insertion of the plug before the plug is inserted to ensure that a suitable surface is provided to support and seal the plug and also to protect the entry portion of the longhole below the plug during adjacent blasting operations.

[0062] The plug according to the first embodiment has been found to be especially effective in retaining the plug at its position within the longhole when the shock wave of a neighbouring blast strikes. The shock wave acts like a hammer on the main body (201) with a force which might drive it from the hole. However, as a result of the tapered engagement between the main body (201) and the retaining means (111), the shock wave results in a substantially lateral expansion force being applied by the retaining means against the bore of longhole. This force provides an additional retaining force of the plug within the hole, thereby preventing it from being “shot” from the hole by the neighbouring explosion.

[0063] The main body (201) and retaining means (202) plug according to the first embodiment may be constructed from suitable metal such as mild steel or cast iron. Alternatively, one or both of the components may be formed from other suitable materials such as certain rigid plastics material. Those skilled in the art will readily recognize that any particular choice of material or materials will provide a set of characteristics which provide certain advantages and disadvantages in a particular application. It is believed that characteristics of the particular mine site such as the nature of the ground will influence the selection of the materials which are best chosen in that instance.

[0064] In a second embodiment of the method of performing the invention, such as might be used in a quarrying application, the blast hole is drilled from the surface above the material. In such an application, it is convenient to fill the blast hole with liquid before the hole is sealed by the plug. A simpler, disposable plug may be inserted within the entry of the hole and sealed by known sealing means. This plug may be removed when its use has finished and discarded.

[0065] In a third embodiment of the method of performing the invention wherein the blast holes are drilled from an upper surface, such as the case of the second embodiment, a re-usable plug is provided having a supply duct and a breather duct. Before filling the blast hole, tubing is inserted into the blast hole and engaged with the supply hole. When it is time to remove the liquid, it may be pumped out through the tube and supply duct.

[0066] In a fourth embodiment, not illustrated, a plug is provided which has features identical to those of the first embodiment, but in addition is provided with a check valve associated with the supply duct. The check valve operates as a one way valve to permit the passage of liquid into the longhole, but not in the opposite direction. The plug is further provided with a release valve adapted to be releasably operable from the tunnel. The release valve seals a drain duct which provides communication between the lower end of the main body and the supply duct, thereby providing means to drain the liquid from the longhole after adjacent blasting is completed. In this embodiment, in addition, a bolt is used to seal the breather tube instead of rod (108). The bolt may be tightened or released from the tunnel by an appropriate portable tool. This embodiment provides the feature that, when installed in the longhole, no pipes or rods extend from it into the tunnel. This is advantageous in certain applications.

[0067] It is believed that the reason for damage being caused to an empty longhole adjoining a neighbouring explosion is because the shock wave from the explosion meets an interface at the longhole between the ore and the air of the empty longhole. It is known that the energy from such shock waves is not transmitted well into the air at such an interface and that a substantial proportion of the energy is reflected or dissipated at the interface. The energy dissipated tends to vibrate the fragile rock surface, dislodging some of it and thereby damaging the longhole. By filling the longhole with a liquid, it is believed that the energy of the shock wave of the explosion is transmitted into and through the liquid far more efficiently and subsequently back into the ore at the other side. This more efficient transfer of energy results in far less energy being expended at the ore to liquid interface and therefore reduces the damage caused by the explosion.

[0068] It should be appreciated that the scope of the present invention need not be limited to the particular scope of the embodiments described above.

[0069] Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 

The claims defining the invention are as follows:
 1. A method of protecting a predrilled, uncharged blast hole comprising the steps of substantially filling the blast hole with a liquid before adjacent blasting is conducted and sealing the entry of the blast hole.
 2. A method of protecting a predrilled, uncharged blast hole as claimed at claim 1 wherein the entry of the blast hole is sealed by inserting a plug into the entry of the blast hole and positioning the plug adjacent to the entry of the blast hole.
 3. A method of protecting a predrilled, uncharged blast hole as claimed at claim 2 wherein liquid is delivered into the blast hole after the plug is inserted.
 4. A method of protecting a predrilled, uncharged blast hole as claimed at claim 2 or claim 3 wherein the entry of the blast hole is lined before it is sealed by the plug.
 5. A plug adapted to seal a blast hole against leakage of liquid comprising sealing means adapted to provide sealing engagement between the bore of the blast hole and the plug, and retaining means adapted to positionally retain the plug within the blast hole.
 6. A plug as claimed at claim 5 wherein the sealing engagement is releasable.
 7. A plug as claimed at claim 6 wherein the sealing means is adapted to provide sealing engagement between the bore of the blast hole and the plug by expansion of the sealing means.
 8. A plug as claimed at any one of claims 5 to 7 wherein the sealing means is responsive to pressure to provide sealing engagement.
 9. A plug as claimed at any one of claims 5 to 8 wherein the sealing means comprises a seal seated in a groove.
 10. A plug as claimed at any one of claims 5 to 9 wherein the sealing means comprises a seal having a ‘U’ shaped cross-section.
 11. A plug as claimed at any one of claims 5 to 9 wherein the sealing means comprises a seal having a circular cross-section.
 12. A plug as claimed at any one of claims 5 to 11 wherein the retaining means is adapted to retain the plug within the blast hole by releasable engagement of the retaining means with the bore of the blast hole.
 13. A plug as claimed at any one of claims 5 to 12 wherein the engagement of the retaining means with the bore of the blast hole is effected by expansion of the retaining means.
 14. A plug as claimed at any one of claims 5 to 13 wherein the retaining means, when in use, is adapted to provide additional retaining force to the bore of the blast hole when additional force is applied to the plug from within that portion of the blast hole enclosed by the plug.
 15. A plug as claimed at any one of claims 5 to 14 wherein the plug is associated with a supply duct adapted to allow the passage of liquid past the plug.
 16. A plug as claimed at any one of claims 5 to 15 wherein the supply duct is provided through the plug.
 17. A plug as claimed at claim 16 wherein the plug is further provided with pressure-communication means adapted to communicate pressure from the supply duct to the sealing means.
 18. A plug as claimed at either of claims 16 or 17 wherein the supply duct is provided with a pressurising nozzle adapted to restrict the flow of liquid through the nozzle and thereby ensure pressure is provided within the supply duct during filling of the blast hole with liquid.
 19. A plug as claimed at any one of claims 5 to 18 wherein the plug is associated with a breather duct adapted to allow the passage of air past the plug.
 20. A plug as claimed at claim 19 wherein the breather duct is provided through the plug.
 21. A plug as claimed at either of claims 19 or 20 wherein the plug further comprises a bleed hole adapted to provide re-sealable communication between the breather duct and an external surface of the plug.
 22. A plug as claimed at any one of claims 22 to 24 wherein a tube is associated with the plug, the tube extending from the plug substantially to the inner end of the blast hole.
 23. A plug as claimed at claim 25 wherein the tube engages the breather duct.
 24. A plug as claimed at claim 25 wherein the tube engages the supply duct.
 25. A plug as claimed at any one of claims 5 to 24 wherein the plug comprises a plug body and the retaining means, the retaining means comprising an annular member received around the plug body, the annular member having a plurality of axially spanning engagement ribs surrounding the periphery of the annular member means.
 26. A plug as claimed at claim 25 wherein the annular member comprises a plurality of part-annular segments.
 27. A plug as claimed either of claims 25 or 26 wherein the retaining means is provided with an inner surface which is tapered.
 28. A plug as claimed at any one of claims 25 to 27 wherein the external surface of the main body is tapered.
 29. A plug as claimed at claim 28 wherein the tapered surface of the main body is adapted to cooperate with the tapered surface of the retaining means to provide expansion of the retaining means.
 30. A plug as claimed at any one of claims 5 to 29 wherein the plug is re-usable.
 31. A plug as claimed at any one of claims 5 to 29 wherein the plug is disposable.
 32. A method of protecting a predrilled, uncharged blast hole substantially as herein described.
 33. A plug adapted to seal a blast hole against leakage of liquid substantially as herein described.
 34. A plug adapted to seal a blast hole against leakage of liquid substantially as herein described with reference to the accompanying drawings. 